This invention relates to the field of liquid filled acoustic lenses and more particularly to a temperature compensated liquid filled acoustic lens. Although not limited thereto, the invention is an improvement to and is disclosed herein in use with the conical liquid filled acoustic lens described in the articles "Opto-Acoustic Feasibility Design of a Conical Acoustic Lens" by Morton Stimler, IEEE Journal of Oceanic Engineering, Vol. OE-3, No. 2, April, 1978, and "Theory and Technical Approach in the Feasibility Design of a Conical Acoustic Lens", by Morton Stimler, 1977 IEEE International Conference on Acoustics Speech and Signal Processing Record. That lens, to be described further for a complete understanding of the subject invention, demonstrated the temperature dependent nature of liquid filled acoustic lenses for proper focusing which the present invention overcomes.
Liquid filled acoustic lenses are used to detect and localize underwater sound waves and their sources such as submarines or torpedoes. They operate on the principle of the refraction of sound waves when passing from one medium to another to focus the sound waves upon suitable transducers.
In order for an acoustic lens to be practical, its focal length must remain nearly constant over its entire expected operating temperature range. This requires that the acoustic index of fraction of the lens fluid, relative to the ambient medium, remain constant over that operating range. In the past, this has been attempted by maintaining the lens fluid temperature constant as the ambient water temperature changed, however merely maintaining the temperature of the lens fluid constant as the temperature of the ambient medium changes does not maintain a constant index of refraction. This is so because the index of refraction of the lens fluid relative to the ambient medium is dependent upon the ratio of the velocity of sound in the ambient medium to the velocity of sound in the lens fluid, which velocities change at different rates and in different magnitudes with temperature. It can be seen that the ratio of these sound velocities and therefore the index of refraction relative to the ambient water will change even though the temperature of the lens fluid is held constant, which results in a change in the focal length of the lens.
Ships utilizing these lenses are subject to temperature changes as they move through the water between a variety of geographical locations, and the depth of the lens below the surface of the water significantly effects a temperature change. It can be appreciated that these temperature changes will therefore result in loss of proper detection and localization of the subject underwater source, and it is therefore highly desirable to have a lens that maintains its focusing properties independent of temperature. Athermal lenses have been devised, but each have limitations. These limitations include restrictions on the type of lens fluids that may be used, operating temperature limits that restrict the ability of the lens to be used in all temperature environments that might be encountered, and predetermination of the temperature to be encountered, which can not always be done. Some methods of compensating for the temperature dependence of the focal length include mounting the hydrophone transducers so as to permit them to be moved to the focal point with temperature changes. These lenses are complex, expensive and time consuming to operate.